Solid tumor models are being used for devising improved therapeutic scheduling of radiotherapy and chemotherapy. Each solid tumor model has properties which made it suitable for particular studies involving the complex tumor-host response, and the dissemination of disease through metastases. Different treatment modalities alone or in varied combinations will be studied for effects at three organizational levels: cellular, organ, and animal, permitting maximum clinical utilization of information obtained from solid tumor models. The gastrointestinal tract, hematopoietic system, liver, heart, and skin are being used to evaluate the effects of single and combined modality therapy on normal tissue. Biological endpoints being used include total tibial marrow DNA, marrow DNA specific activity, and peripheral hematological changes. Changes in DNA content and DNA specific activity of intestinal mucosa are included. Internal changes in the tumor after treatment and during recovery are being evaluated by a number of methods. Studies on the relationship of changes in tumor cell viability and tumor volume are being made because of their clinical relevance. Pathological evaluation of tumor changes include studies on changes in viable tumor tissue surrounding capillaries (tumor cords). Biochemical studies on changes in the concentration of tumor DNA and rate of tumor DNA synthesis are being coordinated with autoradiographic studies of cell kinetics. It has been demonstrated that combined modality therapy can be carried out so as to induce maximum effects on the tumor following host recovery from the previous treatment. High priority is being given to the development of protocols for sequential chemotherapy and radiotherapy. The quantitative information on experimental cancer treatment derived from these studies on animal models should aid the clinician in the design of clinical protocols and contribute to overall national efforts designed to improve the management of patients with the more resistant solid tumors.